The invention relates to a method (referred to as power drive method) of driving a power drive circuit such as drivers for actuators including CDs (compact discs) and CD-ROMs and audio power amplifiers which require a large drive current tolerance, and to a circuit for the method.
In a power drive circuit such as an actuator driver for a CD and a CD-ROM and an audio power amplifier, requiring a large drive current tolerance, bipolar transistors are used as an output element as well as a control element. These bipolar transistors are formed in the form of an integrated circuit (IC) chip together with other elements for the control circuit.
To alleviate some problems encountered in the manufacture of IC chips accommodating both NPN and PNP type bipolar transistors, NPN type bipolar transistors are configured to have a vertical structure in which operating current flows vertically, while PNP bipolar transistors are configured to have a lateral structure in which operating current flows laterally.
NPN bipolar transistors having vertical structures may have a large current tolerance, i.e. allows a large current through it, if it has a small chip area, whereas PNP bipolar transistors having lateral structures cannot, due to the fact that in the latter transistors electric current flow on or near the surfaces thereof under strong influences of the fields that exist on or near the surfaces. This implies that such PNP bipolar transistors have only a limited current amplification factor. In order to construct a PNP bipolar transistor having a large current tolerance, it is therefore necessary to provide the PNP bipolar transistor with a sufficiently large area on the chip, which is, however, a disadvantage from the point of manufacturing cost.
To circumvent this problem, there has been proposed a power drive circuit that utilizes NPN transistors not only as an output power transistor of the power drive circuit but also as a driver transistor for driving the output power transistor.
It is known that, in order to drive a push-pull type output power transistor without crossover distortions associated with it, it is necessary to maintain a constant idling current flowing through the output power drive circuit.
In a conventional power drive circuit as mentioned above, the idling current is normally set to a predetermined level large enough in suppressing any such crossover distortions. The idling current will be left at that level thereafter once it is set, irrespective of the magnitude of an input signal supplied to the power drive circuit or a load connected to the circuit.
However, in a conventional power drive circuit, it is likely that the idling loop falls in a resonating condition, which in turn causes resonation of the output circuit if the idling current has been set to a low level and a large load is connected to the power drive circuit. The resonance results in harmful influences such as erroneous operations of peripheral devices.
In order to prevent such resonance at all times, especially when a large load is connected, i.e. a large output current is required, the idling loop current must be set a high level, making a resonance margin wider. In this case, however, the raised idling current will result in unnecessary energy consumption in the circuit when the input signal has null or very low level.
One way to suppress the resonance of the idling loop is to provide a large capacitor in the idling loop. The capacitor, however, requires a fairly large area, so that the chip for the power drive circuit must be undesirably enlarged and can deteriorate frequency characteristics of the power drive circuit.
It is therefore an object of the invention to provide a power method of driving a power drive circuit free of resonance without any resonance preventive capacitor, thereby suppressing overall power consumption by the power drive circuit to a minimum level.
It is another object of the invention to provide a circuit to realize the method mentioned above.
In accordance with one aspect of the invention, there is provided a method (referred to as power drive method) of driving push-pull type output transistors connected in series between power supplies and operating in response to an input signal, said method comprising a step of controlling the idling current to be passed through the positive and negative sides of said output transistors in response to the level of the output current flowing through said output transistors.
In accordance with another aspect of the invention, there is provided a power drive circuit, comprising:
a positive side output transistor and a negative side output transistor connected in series between power supplies and performing push-pull operations;
an input circuit for receiving an input signal and, in response to said input signal, providing a drive signal to said positive and negative side output transistors;
an idling loop for providing an idling current to said positive and negative side output transistors in accordance with a reference idling current; and
current detecting elements for detecting the drive currents flowing through said positive and negative side output transistors, wherein
said reference idling current is changed in accordance with the detected levels of said drive currents to thereby controlling said idling loop.
Thus, the output transistors performing push-pull operations receive a minimum idling current when no input signal exists or only a low level input signal exists. The idling current will be increased when the output transistors are required to provide a large output current. Thus, the resonance margin of the idling loop is dynamically changed to stabilize the power drive circuit while improving overall cost performance of the drive circuit. It should be appreciated that the invention requires no resonance preventive capacitor having a large capacitor in the idling loop, so that the area required for the power drive circuit on an IC chip can be minimized, allowing not only cost-effective manufacture of the power drive circuit but also provision of desirable frequency characteristics to the circuit.
It is noted that the positive and negative side output transistors are both NPN bipolar transistors, and that both of them have their base and emitter connected with corresponding base and emitter of an associated current detecting NPN bipolar transistor, respectively, so that the reference idling current may be changed based on the current thus detected.
With this arrangement, the drive current (i.e. current through either of the output transistors) can be detected easily by means of the respective NPN current detecting transistors, and the detected current can be used directly in controlling the reference idling current.
It is also noted that the idling loop comprises a multiplicity of base-emitter passages of bipolar transistors, diodes, and resistors that receives current from the current detecting NPN bipolar transistors such that the reference idling current is changed based on the voltage drops across the resistors.
The reference idling current can be easily regulated by making the resistances of the resistors variable. Then the reference idling current can be automatically regulated by the drive current using the variable resistor.